1. Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary battery, more particularly to a non-aqueous electrolyte secondary battery which is most suitable for a thin type lithium-ion secondary battery, and the method of production thereof.
2. Description of the Related Art
Thin type lithium-ion secondary batteries have been commercialized as non-aqueous electrolyte secondary batteries for portable equipment such as cellular phones. This type of battery comprises a positive electrode made of lithium cobalt oxide (LiCoO.sub.2), a negative electrode made of graphitic and carbonaceous materials, an electrolyte solution prepared by dissolving a lithium salt in an organic solvent, and a separator made of a porous film.
Since the portable equipment is becoming increasingly thinner, it is a general requirement to reduce the thickness of batteries. However, it is difficult to produce thin type lithium-ion secondary batteries having a thickness of 4 mm or less. For this reason, card-type lithium secondary batteries having a polymer electrolyte have been proposed, and development for their commercialization has been in progress.
In lithium secondary batteries having a polymer electrolyte, a gel polymer holding a non-aqueous electrolyte solution is usually employed. Consequently, these batteries have larger impedance at the electrode interface and lower lithium-ion conductivity than non-aqueous electrolyte lithium secondary batteries. Further, when the positive- and negative electrodes of the lithium secondary batteries having a polymer electrolyte are made thinner in order to enhance lithium ion conductivity, the amount of the active material of the positive- and negative electrodes is reduced, thereby reducing the energy density.
Thus the lithium secondary batteries provided with a polymer electrolyte have a problem that they are inferior in volumetric energy density and large-current characteristics to thin type lithium secondary batteries which is impregnated with a non-aqueous electrolyte solution in a liquid state.
Japanese Patent Disclosure (Kokai) No. 10-177865 describes a lithium ion secondary battery which comprises a positive electrode, a negative electrode, a separator holding an electrolyte solution and having surfaces facing the positive electrode and the negative electrode, and adhesive resin layers disposed on the surfaces of the separator, which adhesive resin layers comprise a mixed phase of an electrolyte solution phase, a polymer gel phase containing an electrolyte solution and a solid polymer phase, said adhesive resin layers joining the positive- and negative electrodes with the adjacent surfaces of the separator. A claim of Japanese Patent Disclosure (Kokai) No. 10-189054 describes a method of producing a lithium ion secondary battery which comprises processes of forming positive- and negative electrodes on corresponding current collectors, coating a separator with a binder resin solution in which polyvinylidene fluoride is dissolved as a main component, forming a battery laminate by stacking the electrodes on the separator and drying the closely contacted electrodes and separator to vaporize the solvent, and impregnating the laminate with an electrolyte solution.
In these lithium ion secondary batteries, however, adhesive resin layers are interposed between the positive electrode and the separator and between the negative electrode and the separator. Consequently, these lithium ion secondary batteries have increased internal resistance, and inferior cycle life and large-current discharge characteristics.
The present invention relates to a non-aqueous electrolyte secondary battery and the production method thereof. More specifically, the non-aqueous electrolyte secondary battery of the present invention has an electrode assembly into which non-aqueous electrolyte is impregnated as a solution, wherein not only the thickness of the battery can be reduced by a simple means but also capacity, large-current characteristics and cycle life can be improved.
According to an aspect of the present invention, there is provided a production process of a non-aqueous electrolyte secondary battery which is equipped with;
a positive electrode including a positive electrode active material and a binder as its constituents;
a negative electrode including a negative electrode active material and a binder as its constituents;
a separator interposed between the positive electrode and the negative electrode; and
a non-aqueous electrolyte solution;
wherein the above production process comprises steps of;
preparing an electrode assembly by interposing a separator between a positive electrode and a negative electrode;
impregnating the electrode assembly with an organic solvent which can dissolve the binder of the positive and/or negative electrodes;
bonding the positive electrode and the separator together and bonding the negative electrode and the separator together by drying the electrode assembly; and
impregnating the electrode assembly with a non-aqueous electrolyte solution.
Further, the non-aqueous electrolyte secondary battery according to the present invention comprises;
an electrode assembly consisting of a positive electrode, a negative electrode, and a separator; and
a non-aqueous electrolyte solution;
wherein a binder is contained obeying a specific distribution in at least part of each constituent layer of the electrode assembly consisting of a laminate of a positive electrode, a separator and a negative electrode, the distribution of the binder having a concentration gradient such that the binder concentration measured in the cross-sectional direction of the electrode assembly has a minimum at the center of the separator and becomes greater toward the interfaces between the positive- and negative electrodes and the separator, thereby forming a U-shaped distribution.
We explain first invention as follows. The first invention is a method for manufacturing a non-aqueous electrolyte secondary battery including an electrode assembly that include a separator between a positive electrode having a first binder material and a negative electrode having a second binder material comprising;
a step of impregnating said electrode assembly with an organic solvent to dissolve at least of said first and second binders,
a step of drying said electrode assembly for bonding said positive electrode and said separator and for bonding said negative electrode and said separator together.
We explain second invention as follows. The second invention is a non-aqueous electrolyte secondary battery comprising;
a positive electrode including a positive electrode active material, conductive material and a first binder,
a negative electrode including a carbonaceous material which adsorbs or desorbs lithium ions and a second binder,
a separator interposed between the positive electrode and the negative electrode, and
a non-aqueous electrolyte solution,
wherein a concentration gradient of the sum of the first and second binders is a continuous curve such that the binder concentration has a minimum at the center of the separator, becomes gradually greater toward the interfaces between the position electrode and the negative electrode and is saturated to the maximum at the centers of the negative electrode and the positive electrode.
We explain third invention as follows. The third invention is a non-aqueous electrolyte secondary battery comprising;
an electrode assembly including of a positive electrode, a separator and a negative electrode; and
a non-aqueous electrolyte solution;
wherein a binder is contained, obeying a concentration distribution, in at least part of each the positive electrode, the separator and the negative electrode, the concentration distribution of said binder having a concentration gradient such that the binder concentration measured in the cross-sectional direction of said electrode assembly has a minimum at the center of said separator and becomes greater toward the interfaces between said positive electrode and said separator and between said negative electrodes and said separator, thereby forming a U-shaped distribution.